Electrophotographic printing processes typically involve creating an image on a photoconductive surface, applying an ink having charged particles to the photoconductive surface, such that they selectively bind to the image, and then transferring the charged particles in the form of the image to a print substrate.
The photoconductive surface may be on a cylinder and is sometimes termed a photo imaging plate (PIP). The photoconductive surface is selectively charged with a latent electrostatic image having image and background areas with different potentials. For example, an electrostatic ink composition comprising charged toner particles in a carrier liquid can be brought into contact with the selectively charged photoconductive substrate (e.g. paper) directly or, more commonly, by being first transferred to an surface. The charged toner particles adhere to the image areas of the latent image while the background areas remain clean. The image is then transferred to a print intermediate transfer member, which can be a soft swelling blanket, and then to the print substrate.
Two methods can be used when multi-color images are to be printed. Both involve the generation of multiple separations (i.e., single color partial images) that, when superimposed, result in the desired print image. Each of the images is separately generated on the photoconductive surface, transferred to the intermediate transfer member and then to the final substrate. In some systems, the images are separately transferred from the intermediate transfer member to the final substrate. In these systems, the images on the intermediate transfer member are each transferred in turn to, and are superimposed, in registration, on the final substrate; such systems are sometimes termed multi-shot systems. In other systems, referred to herein as one-shot systems, the images are separately formed on the photoconductive surface and are transferred in turn to the intermediate transfer member in registration and superposition thereon, to form a print image. The superimposed images in the form of a print image are then transferred together to the print substrate.
In some one-shot systems, the same photoconductive surface is used to generate the separations sequentially. In other systems a plurality of photoconductive surfaces are present, each of which generates an image of a different color, the plurality of images being superimposed on the intermediate transfer member.